I think Jim was ready to move on to other things. Just like in a career, you don't want to stick to the same thing you want to try new things (well at least you SHOULD want to). Jim was gong towards more complex projects towards the end of his life. The conceptual work of Dinosaurs, the Storyteller, Jim Henson Hour, and Teenage Mutant Ninja Turtles were all examples of his change in direction within the muppet universe. Do I think Kermit would still be making cameos and a special here or there, yes. I guess we'll never really know.